1. Field of the Invention
This invention relates to coating compositions formulated to cure by free radical mechanism useful for coating the exterior surface of cleaned and prepared metal tubing and other metal substrates and the process for achieving application and curing of the coating compositions. The tubing and other metal objects to which the invention is applicable may be comprised of cold rolled steel, hot dipped galvanized steel, electro galvanized steel, Galvalume and aluminum. More particularly this invention relates to coatings formulated to cure by free radical mechanism and which are polymerized using an electron beam generator. The electron beam generator creates energy that is absorbed through inelastic collisions between high energy electrons emanating from an acceleration chamber of the generator and the electrons associated with the components of the coating. The coating component electrons are excited or ionized and both forms can lead to initiation of radical polymerization in which the coating does not block or stick. The coating function is decorative as well as utilitarian and complies with the specific properties required depending upon the specific market and requirements of the tube or product producer.
2. Description of the Prior Art
Coatings have long been available for a wide variety of production metal tube. Coatings include those containing solvents and dry by solvent evaporation and or oxidation. Other types of coatings that have more recently come into use include, but are not limited to, water reducible, powder and radiation light curable coatings. These coatings, including water reducible, powder and light reactive coatings, have inherent limitations:
Water reducible coatings are sensitive to freezing and require energy to remove water from the film prior to curing. In some water reducible coating formulations, a coalescing solvent is present which may cause the coating to be more water sensitive. In addition, the coalescing material may be classified as a volatile organic compound, impacting air quality. Typically, metal preparation is more critical when using water reducible coatings as compared to solvent type coatings. The cleaning stages, chemicals, concentrations, temperature and dwell time vary, depending on the metal from which the tubes are composed, line speeds and conditions as well as market requirements determined by the tube or product producer.
The light radiation curable coatings have limited light emitting element, i.e., bulb life, and are sensitive to breakage and have difficulty curing opaque coatings. The light reactive coatings crosslink only to the extent that spectral radiation energy penetrates the film and activates photoinduction inducing free radical polymerization. If the light emitting element or bulb is old, dirty, or out of focus, cure of the coating composition may be significantly retarded. If the light reflector of the light radiation aparatus is allowed to become dirty or contaminated, cure of the coating composition may also be significantly retarded. Light radiation curable opaque coatings are more difficult to cure compared to clear coats and given specific colors, opacity, film thickness, available energy, or line speeds, many of such coatings cannot be cured by light radiation.
Powder coatings require higher temperatures and/or longer dwell times in order to cure the coating. The Powder coatings higher temperature requirements can cause problems with an interior tube coating such as degradation or fire. In addition the tube is more prone to bending or flexing at high temperatures while being conveyed which may result in handling or production down time problems. The high temperatures and increased energy associated with powder coatings can mean higher costs. Color changes are more critical with powder coatings because, being a powder, one color can not blend into another. When powder coatings are utilized, any contamination present on the tube or in the coating composition shows up as a separate color spot.
Metal tubes are sold into a variety of markets and products that include but are not limited to, mechanical applications, fencing, furniture (predominately outdoor furniture, liquid transmission applications and walking canes). Cleaning procedures will vary depending on the metal being processed, coating being used, cleaning chemical, metal pretreatment chemical, chemical temperatures, line speeds and processing equipment condition. Coatings used for coating tubular goods and other metal substrates are usually high or medium gloss coating formulations that provide both decorative as well as protective qualities. Film thicknesses vary from about 0.5 mil to about three mils depending on the application and market. Line speeds for tube coating processes are typically from a low of about seventy five feet per minute to as high as typically six hundred feet per minute. The tubes being coated may variety in diameter and wall thickness and are typically cut to various lengths depending on end user or customer requirements.
The methods of coating application for tubular goods may include air atomized, airless atomized and air assist-airless atomized coating application systems. These various coating application systems may also employ heated coatings and may be operated in both manual and automatic mode. Other methods are vacuum coaters, continuous coaters, flood and wipe and flow coaters. These methods may also include variations and combination such as for example heating the coating and/or employing automatic viscosity control means or other system variations currently in the public domain. In some cases both the interior and exterior of the tubes are coated. Typical tube cross-sectional tube configurations include round, square or rectangular tube stock. The coatings may consist of clears, primers, sealers or color applied directly to the prepared metal.
Conventional Tube Coating Processing
A conventional tube coating process typically consists of the following processing steps:
1. Coil entry
This step represents the introduction of coiled metal strip material into a coated metal tube production line. Typically, a coil of metal strip material is mounted for rotation and the strip material is moved endwise from the coil and conducted to tube forming apparatus.
2. Tube formation
This step represents essentially linear or endwise transfer of metal coil tubing stock into tube forming rolls where the tubing stock is formed to the desired configuration and welded.
3. Tube cleaning . . . usually heated cleaning solution
The tube is cleaned in this step and typically uses a chemical/water solution to remove the metal surface contaminates.
4. Rinse . . . water or water chemical rinsing solution combination
This step removes any excess water/chemical solution.
5. Chemical pretreatment . . . usually a heated solution
A chemical pretreatment solution is applied to react with and modify the metal surface of the tube to produce a surface suitable for painting, to enhance coating adhesion and to provide for corrosion resistance. Specific chemicals are applied for surface pretreatment, depending on the characteristics of the metal, production conditions and tube end use requirements.
6. Rinse/dry
The tube is given a rinse and is dried. It should be noted that some production lines incorporate additional cleaning and chemical pretreating steps before the tube moves to the coating application area.
7. Application of coating
The coating is typically applied by spray or vacuum coater method but other application equipment such as continuous coater, airless, air assist, air spray have been employed some using heaters to elevate the coating temperature for example.
Electrostatic equipment is the primary equipment used to apply powder coatings. Most coating systems are automatic.
8. Cure coating
Present art is practiced utilizing devices sufficient to dry or cure various types of coatings as described. They include but are not limited to: induction heating, gas fired ovens, ultra violet curing devices, such types as are available from Fusion UV Systems, Inc. located at 910 Clopper Road, Gaithersburg, Md. or Aetek Internal, Inc. a Subsidiary of Fusion UV Curing Systems. Other types of curing devices for tube coatings may include Infrared, high velocity gas or electric devices.
9. Cool on quench step may be required due to the high tube temperature that is required when a powder coating is applied to a metal substrate and cured.
10. Cut and/or band Tube
After the tubing has been properly coated, it is readied for shipment to customers by gathering finished tubing into bundles containing desired numbers of tubes and applying retainer bands to the bundles so that the tubes can be efficiently handled and shipped. A conventional coated tubing manufacturing process typically consists of the following steps:
1. Coil entry
This step represents the introduction of metal coil into the production line.
2. Tube formation
This step represents transfer of the coil into the tube forming rolls where the tube is formed and welded.
3. Cleaning the of the tube
After the tube is formed, it is cleaned and rinsed. The cleaning system typically utilizes a chemical/water solution that is heated. In most cases tanks are designed to clean the pipe utilizing a spray or flood procedure to apply the cleaner to the metal as it passes through the tank or tanks. A water rinse tank or tanks is used to assure that the cleaner residue is removed. The water rinse tank may also be heated. If the metal is not cleaned properly, paint adhesion problems will occur.
4. Rinse
This step removes any excess water/chemical cleaning solution.
5. Chemical Pretreatment
The chemical pretreatment or conversion coat step provides a protective layer to the substrate. This chemical treatment is designed to react with and modify the metal to produce a surface suitable for painting and to enhance the paint adhesion. This conversion coat also provides protection to the substrate from exterior corrosion.
6. Rinse
This step may consist of one or two rinse stages using water/chemical and or a water rinse. As the tube continues moving forward from the final rinse tank, it is dried and moves into the chemical coating section.
7. Application of paint or coating
The application of the paint or coating to the metal is accomplished by passing the tube through the paint coater. The paint coater may consist of spray, flood and wipe, vacuum or continuous coaters or other typical methods of application known to the industry. Conveyor rolls typically support the tubing as it moves through or past the coating application equipment area. Settings and conveyor speeds must be controlled for consistent application of paint film. The correct paint film will vary depending on the specification of each individual paint and product requirement. Before any liquid paint, containing volatiles is used in the paint coater, it must be properly mixed to achieve the specified application viscosity. Periodic checks and viscosity adjustments must be made to allow for changes due, for example, to solvent evaporation.
8. Curing the paint
After the paint has been applied, the continuous tube then moves directly into the drying unit, curing ovens, or light radiation unit. In the case of powder, the meltflow-cure process can occur as a result of preheating the tube to a high temperature with, for example, an induction heater after the final rinse. The curing equipment temperatures must be set at a level to achieve a peak metal tube temperature, or light radiation energy, specified by the coating supplier for a specific paint and line speed. With the proper film thickness, line speed and metal temperature, or radiation achieved, the curing step is complete.
9. Cooling or quench step. This step may be required for cooling the coated tube due to the high heat that is necessary for curing of powder coatings.
10. Cutting the tube (in line process only) and banding.